Conventional printed wiring boards include those having conductive layers 911 to 914 built up successively, as shown in FIG. 42. The conductive layers 911 to 914 are electrically connected to one another via interconnecting through holes 931 to 933. Insulating layers 921 to 923 are interposed between the conductive layers 911 to 914, respectively.
The conductive layer 911 is a component-connecting layer on which an electronic component 961 is mounted and conducts electric currents in and out of the electronic component 961. The conductive layer 911 which is one of the outermost layers and the electronic component 961 are electrically connected to each other by bonding wires 962. The conductive layer 914 which is the other outermost layer serves as an external connecting layer for connecting external connecting terminals 97 and leading electric currents in and out of a printed wiring board 941. The internal conductive layers 912 and 913 are electric current transmitting layers for transmitting internal currents of the printed wiring board 941.
Next, the method of manufacturing the above printed wiring board will be described.
First, as shown in FIG. 43, conductive layers 912 and 913 are formed on the upper side and lower side of an insulating layer 922 respectively. Further, interconnecting through holes 932 are formed through the insulating layer 922, and the wall of each interconnecting through hole 932 is covered with a metal plating film 95. A resin 92 is then packed in the interconnecting through holes 932.
Next, an insulating layer 921 and a copper foil are laminated on the upper side of the insulating layer 922, while an insulating layer 923 and a copper foil are laminated on the lower side, followed by etching of the copper foils to form conductive layers 911 and 914.
Subsequently, as shown in FIG. 44, interconnecting through holes 931 and 933 are formed through the insulating layers 921 and 923 to expose the surfaces of the internal conductive layers 912 and 913, respectively.
Then, as shown in FIG. 42, a metal plating film 95 is formed on the walls of the interconnecting through holes 931 and 933, and external connecting terminals 97 are bonded onto the surface of the outermost conductive layer 914.
Thus, the printed wiring board 941 can be obtained.
By repeating the procedures shown in FIGS. 43 and 44, the number of conductive layers to be built up in the printed wiring board 941 can be increased. The thus obtained printed wiring board has insulating layers and conductive layers built up alternately both on the upper side and on the lower side of the center insulating layer 922. Therefore, an even number of conductive layers are formed according to the above method.
However, the conventional method of manufacturing printed wiring boards as described above is not suitable for building up an odd number of conductive layers, although it can build up an even number of conductive layers efficiently.
To describe, for example, a case where a printed wiring board having five conductive layers 910 to 914 built up, as shown in FIG. 45, is manufactured, the second to fifth conductive layers 911 to 914 are built up first, as shown in FIG. 46, in the same manner as described above, except that the conductive layer 914 is an unpatterned copper foil.
Next, as shown in FIG. 47, the conductive layer 914 is removed completely, and then interconnecting through holes 931 are formed, as shown in FIG. 48, followed by formation of a metal plating film 95 on the wall of each through hole 931. Subsequently, as shown in FIG. 49, prepregs are laminated and press-bonded to form insulating layers 920 and 924. Conductive layers 910 and 914 are then formed on the surfaces of the insulating layers 920 and 924 respectively, followed by formation of interconnecting through holes 930 and 933 through the insulating layers 920 and 924 respectively, as shown in FIG. 50. A metal plating film 95 is formed on the walls of the through holes 930 and 933, as shown in FIG. 45.
As described above, when a printed wiring board having an odd number of conductive layers is manufactured, it is necessary, in order to prevent warping of the press-bonded printed wiring board from occurring, to carry out, after formation of the internal conductive layers 911 and 914, the procedure of removing the conductive layer 914. Thus, the conventional method requires wasteful a procedure and is an extremely inefficient manufacturing method. Further, the insulating layers formed are too thick to meet the purpose of achieving downsizing of printed wiring boards.
Under such circumstances, it can be considered to form an insulating layer 920 and a conductive layer 910 only on one side of the insulating layer 921. In this case, however, warping of the printed wiring board can occur in the step of press-bonding a prepreg for forming the insulating layer 920.
Meanwhile, in a multilayer build-up type printed wiring board, the internal insulating layers 921 and 923 to be embedded in it are resins, so that they have high coefficients of water absorption of 0.5 to 1.0% and have high water contents. The water is vaporized naturally with passage of time to assume the form of water vapor which collects mainly, for example, between the insulating layer 921 and the adjacent insulating layers 922 and 920 and between the insulating layer 923 and the adjacent insulating layers 922 and 924.
Accordingly, it is likely that the interlayer adhesion is lowered and that the layers undergo delamination. Particularly, the greater the number of layers laminated, the greater becomes the number of water-containing internal insulating layers, and the higher becomes the tendency of interlayer delamination.
Meanwhile, referring to manufacturing of printed wiring boards, there is a method invented by us previously and disclosed in Japanese Patent Application No. Hei 8-21975. That is, as shown in FIG. 51, a conductive layer is formed on each insulating layer in step S91, and then interconnecting through hole-forming through holes are defined in each insulating layer in step S92. Steps S91 and S92 are repeated corresponding to the number n of insulating layers to be laminated. Next, in step S93, the number n of insulating layers are laminated via an adhesive material and positioned such that the through holes in the respective layers may communicate with one another to constitute interconnecting through holes. In step S94, the adhesive material is melted by heating and the like, and the layers are press-bonded together to form a multilayer substrate. In step S95, a conductive material, such as a solder and the like is packed into the interconnecting through holes to impart conductivity to them. Thus, a printed wiring board is obtained.
However, in the conventional method of manufacturing printed wiring boards described above, interconnecting through hole-forming through holes must be defined in each insulating layer independently. Accordingly, the method requires intricate procedures of defining through holes. Further, the through holes must be positioned. Particularly, with the reduction in the size of the interconnecting through holes, it is becoming difficult to carry out accurate registration of the through holes.
Meanwhile, in a multilayer printed wiring board, pads for connecting external terminals such as solder balls are provided on the outermost layer. In this case, the interconnecting through holes must be electrically connected with the pads by connecting circuits. However, the connecting circuits which occupy a large surface area are a hindrance in achieving high-density packaging on the substrate surface. Particularly, in a multilayer printed wiring board, it is necessary to form high-density wiring on the uppermost surface. Further, large amounts of electric currents must fed in and out through the external connecting terminals.
The present invention is directed, in view of the problems inherent in the prior art described above, to provide a printed wiring board which can improve electrical properties of multilayered wiring boards and a method for manufacturing the same. Particularly, it is a first objective of the present invention to build up an odd number of conductive layers efficiently with no warping. A second objective of the present invention is to prevent delamination of layers. A third objective of the present invention is to form interconnecting through holes at accurate positions. A fourth objective of the present invention is to carry out transference of a huge amount of electrical information through solder balls for external connection and also to achieve high densification of surface packaging.